That would be great but take into account champions on rotation. Personally, I haven't purchased each champion in the game and there are a few I am not interested in getting. Some times in ARAM I do get these champions and can not get rid of them so in cases like that, it would be a complete waste for me to gain points for them. I won't take that champion seriously nor will I be persuaded to buy them soon after.
The following idea might be to just give points to people who have the champion they got but that would just be calling for retaliation from players about "unfair game mechanics" or what have you. And I would agree.
ARAM really doesn't need champion masteries implemented into it. I see it more as a fun game more, one you don't take too seriously. It is also pretty luck based and totally random so that just adds an air of unexpectedness and some games are won more easily than others.